Bird early specification and development

Bird bio 405

Where and when does fert happen in birds

happens in the birds oviduct before the egg white and shell are developed and it remains there until late blastula stages

Describe functions of amnion, Allantois, yolk sac and the chorion

1. Amnion: protected embryo in fluid filled cavity
2. Allantois: waste disposal
3. Yolk sac: nutrients
4. Chorion: gas exchange

What is teh marginal zone

Thin layer of cells between area pellucida and and thick area opacity edges of blastoderm

Hypoblast formation

1. Extra cells under epiblast form posteriorly in the area opaca epiblast called Kollers sickle
2. The cells at the front of the kollers sickle are Precoursors of secondary hypoblast
3. These precursors connect with hypoblast if islands creating blastoceaol like spaces as well as forming the hypoblast

what kind of egg is a chick

Telolecithal: small disc of yolk free cytoplasm called the blastodisc

what makes the subgerminal cavity

blastoderm cells absorb water and then secrete water making the space between the blastoderm and yolk.

How is the area pellucida formed and what’s it do?

The are pellucida is the 1 cell thick layer on the blastoderm, and this is where most of teh embryo is formed

Where does the embryo come from

blastoderm forms two layers the upper layer is called the epiblast and the lower layer is called the hypoblast where the epiblast forms the entirety of the embryo and hypoblast forms extra embryonic membranes and secrete signals that specify migration of the epiblast cells

Primitive streak

1. Where gastrulation takes place
2. Equivalent to the blastopore lip
3. As cells form primitive streak they create a depression called primitive groove within the streak, homologous to blastopore
4. Begins at posterior end
5. It elongates via convergent extension doubling the length and halving the width
6. Serves as a gateway for cells into deep layers of the embryo
7. Defines major axis by extending posterior to anterior

Hensens node

1. Center of hensens node has a funnel shaped depression that cells enter into embryo with forming notochord
2. At the head of the primitive streak
3. Functional equivalent of dorsal lip
4. Gives rise to prechordal mesoderm, notochord,
5. Cells that Ingres form the heart and kidneys
6. Cells that dont ingress and are close to the streak form neural plate and farther ones form epidermis

Elongation of primitive streak

1. Cells enter through primitive streak and under go EMT via ingression
2. Streak elongates to head as cells continue to migrate inward towards the center (convergent extension)
3. Secondary hypoblast migrates inward from pmz which also helps elongate streak
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Regression of primitive streak

1. Retraction of hensons node leaves the mesoderm offset from the epiblast which is set to become the notochord


1. Notochord: a tube used to specify neural tube formation
2. The neural tube is used as a signaling point to create the neural groove which acts as a precursor to the neural tube
3. Somites form in the esoteric and are blocks of mesoderm that lead to skeleton and muscle

Formation of endoderm and mesoderm

1. Establishment of germ layer needs to happen prior to gastrulation
2. Cell specification is done after migration through PS and controlled through inductive influences
3. Cells migrating anteriorly an form endoderm and head mesoderm as well as notochord
4. Cells passing through posteriorly give rise to the majority of the mesodermal tissues
5. First cells through he sends node form endoderm of foregut
6. Next cells passing through form chordamesoderm


1. Head process and notochord
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Axis specification and the avian organizer

1. as the egg is developing in the hen it is rotated a bunch which shifts the lighter components to the top due to gravity creates the PMZ, where the primitive streak begins, and lies below one side of blastoderm
2. Unknown how blastoderm forms pmz, however blastoderm with marginal zone is sufficient to form primitive streak
3. The pmz acts as a Vg1 expression center which is apart of the the signaling pathway


1. Needed in formation of the pmz which is the starting point for elongation of the primitive streak
4. Hensons node acts the chord in expression pathway


1. Chord in expression works in elongation of the primitive streak moving it antioriorly
5. Beta catenin is expressed in entire marginal zone while vg1 is only expressed posteriorly making the pmz

Left right axis formation

1. Primarily retaliated by paracrine factors nodal and transcription factor Pitx2
2. SHH protein activates cerberus(head forming factor) which acts with BMP stimulating nodal which regulates the left side of the body. Nodal activates Pitx and also acts as a repressor for snail while a signal called lefty acts as a barrier for cerberus keeping expression on he left side of the body
3. Experiments that change expression of nodal and Pitx2 causes asymmetry in balances and randomizes it causing problems.